High Temperature Tensile and Creep Properties of Some Platinum Alloys

Effects of Additions of Palladium

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Some time ago Dr. G. Reinacher of the Degussa Research Laboratories reported (1, 2) that pure platinum and platinum alloyed with 4 per cent of palladium were more ductile than rhodium-platinum alloys between 700° and 1100°C. In a recent paper (3) he describes the results of experiments designed to confirm these observations and to assist in the development of higher strength alloys that are more ductile than rhodium-platinum in the intermediate temperature range. The compositions of the three palladium-bearing alloys selected for testing were well within the solid solution range and the alloys were not likely, therefore, to be subject to the intermediate temperature brittleness which has previously been tentatively associated by the author with precipitation under the influence of a tensile stress.

The specimens were tested in the form of wire 2 mm diameter and the elongation at fracture was determined over a 50 mm gauge length.

The tests were carried out in air. Some of the more important results are tabulated below.

These values show that a 5 per cent addition of palladium can be made to a 5 per cent rhodium-platinum alloy without loss of high temperature strength and with a slight improvement in ductility at temperatures up to 1250°C. Palladium additions of 10 per cent have an adverse effect upon both strength and ductility. Although the alloy containing 7 per cent of rhodium and 3 per cent of palladium has fairly good compromise properties, its strength and ductility at 1500°C are very much inferior to those of the simple 10 per cent rhodium-platinum alloy. Microscopic examination showed that all the palladium bearing alloys exhibited intercrystalline cracking when creep tested at 900°C. This cracking did not occur in the alloys containing 3 and 5 per cent of palladium tested at 1250°C although the 10 per cent palladium alloy exhibited this type of failure even at 1500°C.

Included in the table are some results for pure palladium taken from another recent paper by the same author (4). Here Dr. Reinacher discloses the existence of a ductility maximum in pure palladium at 500°C. Elongations of the order of 95 per cent were observed when palladium was tensile tested at 500°C. At 400 and 600°C the elongation values were only 30 per cent. Similar effects were observed during creep testing, the elongations at 500°C ranging from 95 per cent for failure in 5 hours to 40 per cent for failure in 100 hours. At 700°C, however, the greatest elongation reported was about 23 per cent. This decline in ductility with time and temperature above 500°C was shown to be caused by oxidation which spread from the surface layers and caused tearing at the grain boundaries. Above 870°C, the dissociation pressure of the oxide, strings of fine porosity developed along the grain boundaries.

Temp °C

Pure Pd

5% Rh 5% Pd

5% Rh 10° Pd

f100

e%

f100

e%

f100

e%

900

1200

4

2480

28

2240

25

1250

170

2

838

88

738

24

1500

—

—

326

66

341

59

Temp°C

7% Rh 3% Pd

5%Rh

10% Rh

f100

e%

f100

e%

f100

e%

900

3540

50

2480

18

3900

22

1250

1020

72

808

62

1105

59

1500

341

39

341

71

483

66

f100 = stress in pounds per sq. inch for a 100 hour life

e = elongation per cent at fracture on a 50 mm gauge length

These results explain the poor creep properties of palladium, but are not easily reconcilable with the observations of the ternary alloy research. It is conceivable that palladium facilitates the entry of oxygen into rhodium-platinum alloys but the effect only becomes appreciable when 10 per cent of palladium is present and when the temperatures exceed 1250°C. Any advantages resulting from the addition of palladium to rhodium-platinum alloys appear to be marginal, and only apparent over a limited temperature range. Outside this range the effects are detrimental. The results of some recent American investigations (5) indicate that platinum alloys containing up to 40 per cent of rhodium display high strength and appreciable ductility at 927°C. At 1450°C this alloy was able to withstand a stress of 800 pounds per square inch for 100 hours. Limitations of the apparatus did not permit of elongation measurements at this temperature. These three researches do, in fact, confirm that no alloys having high temperature properties superior to those based on the rhodium-platinum system have yet been developed. A. s. D.